I'm also very young, but I've had days where, by lunch, I can't remember what I ate for breakfast. Or, after putting on a jacket and going outside in the cold, I can't remember what shirt I'm wearing under the jacket. Or I've put the box of cereal in the refrigerator and the jug of milk in the pantry. And whenever I look at the clock, I have to say the time out loud if I want to remember it for more than two seconds. The list goes on and on. And finding words is always difficult!
It can be scary to feel like your intelligence is slipping away. It's an awful feeling. But I don't think it's dementia. On the days when I'm feeling really good it's like my brain is just about fully functioning again. So I know that my cognition is still there, it's just buried under all that fatigue!
I've lived with the poor recall and thinking problems for many years of having ME/CFS. Avoiding PEM, and taking B vitamins, zinc, and sunflower lecithin have all helped to improve my memory and thinking quality somewhat.
The upside is that I've developed a greater sense of empathy for people with memory and other cognitive problems since I know how it feels.